1. Field of the Invention
The present invention relates to the field of radio-frequency (RF) signal distribution, and in particular to an apparatus and method for distributing RF signals through low bandwidth infrastructures.
2. Description of Prior Art
The demand for wireless communications continues to grow rapidly. The need for efficient and low-cost systems for distributing radio frequency (RF) signals is a direct consequence of this growth. Distribution of RF signals is particularly difficult in areas with many natural and man-made obstacles which scatter or absorb RF radiation. For example, the problem of RF distribution is especially acute inside and around building structures.
Current in-building distribution systems consist of two major parts. The first is a set of antennas and associated accessories required for re-transmitting the RF signals inside buildings. The second is a cabling system, e.g., an optical fiber network, used for interconnecting the in-building antennas with a main antenna. The latter is usually installed on top of the building or at some location where the external RF signals can be easily intercepted. In cases where the RF signals are supplied via a high transmission bandwidth cable the main antenna can be replaced by a direct interface with the RF network, e.g., in the basement.
The most effective manner of distributing RF signals inside or around buildings is to use an existing or standard in-building cable infrastructure. Unfortunately, several obstacles prevent this approach. The major problem is related to the frequency bands used for transmitting RF information signals. Cellular communications presently utilize a carrier frequency around 1 GHz. For, example, the AMPS protocol uses the bandwidth from 824-894 MHz and GSM is transmitted between 890-960 MHz. Recent legislation has allowed PCS services to move to even higher frequencies (e.g., 1,850-1,990 MHz). In comparison, the standard in-building cabling such as unshielded or shielded twisted pair (UTP and STP) used for local area networks (LAN), telephone cables, multi-mode optical fiber links, and power lines are limited to much lower transmission bandwidths. For example, category 5 (10 base T) UTP cable has signal loss and cross talk properties that limit the bandwidth to approximately 0-100 MHz for distances &lt;100 m. Although these parameters suffice for LAN applications, they are clearly inadequate for the delivery of PCS signals to and from remote antenna sites.
In the related patent application (patent application No. 08/871556), we disclosed a novel system and a method for transmitting a radio frequency (RF) signal in an RF bandwidth over a low bandwidth medium, e.g., in-building cabling, which has a transmission bandwidth below the RF bandwidth. The original RF signal is recovered at the desired location by using this invented system and method. This system and method is both cost effective and reliable.
Additional issue arises when certain PCS service, such as a cellular communication service, is required to be limited to a desired location. One example of such situation is: An employer gives employees cellular telephones to be used at a work site. Another example of such situation is: A shopping mall lends customers cellular telephones to be used during shopping. In these situations, it becomes a desired feature of the cellular telephones to be operable only within the designated locations, such as the work site or the shopping mall. Such issue is not resolved in the systems or the methods provided in the prior art or the related patent.